1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an active matrix (AM) device, and an AM device containing the composition. In particular, the invention relates to a liquid crystal composition having a negative dielectric anisotropy, and to a device having an IPS (in-plane switching) mode or a VA (vertical alignment) mode containing the composition.
2. Related Art
In a liquid crystal display device, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth and AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source includes a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes a relationship between the general characteristics of the two. The general characteristics of the composition will be explained further based on a commercially available AM device. A temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or more and a desirable minimum temperature is approximately −10° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of a LiquidCrystal Composition and an AM DeviceGeneral CharacteristicsGeneral CharacteristicsNo.of a Compositionof an AM Device1Temperature range of a nematicUsable temperature rangephase is wideis wide2Viscosity is small1)Response time is short3Optical anisotropy is suitableContrast ratio is large4Dielectric anisotropy isThreshold voltage is low,positively or negatively large.electric power consumptionis small, and a contrastratio is large5Specific resistance is largeVoltage holding ratio islarge and a contrast ratiois large6It is stable to ultravioletService life is longlight and heat1)A liquid crystal composition can be injected into a cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the device. A product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to maximize the contrast ratio. A suitable value of the product depends on the kind of operation mode. In a device having a VA mode, a suitable value is from approximately 0.30 μm to 0.40 μm. In a device having an IPS mode, a suitable value is from approximately 0.20 μm to approximately 0.30 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio of the device. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance at room temperature and also at a high temperature is desirable after it has been used for a long time. A stability of the composition to an ultraviolet light and heat relates to a service life of the liquid crystal device. The service life of the device is long when the stability is high. These characteristics are desirable for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
A composition having a positive dielectric anisotropy is used in an AM device having a TN mode. A composition having a negative dielectric anisotropy is used in an AM device having a VA mode. A composition having a positive or negative dielectric anisotropy is used in an AM device having an IPS mode. Examples of a liquid crystal composition having a negative dielectric anisotropy are disclosed in the following patent documents, JP 2001-262145 A, JP 2001-115161 A.
A desirable AM device is characterized as having a usable temperature range that is wide, a response time that is short, a contrast ratio that is large, a threshold voltage that is low, a voltage holding ratio that is large, a service life that is long, and so forth. Even one millisecond shorter response time is desirable. Thus, the composition having characteristics such as a high maximum temperature of a nematic phase, a low minimum temperature of a nematic phase, a small viscosity, a large optical anisotropy, a positively or negatively large dielectric anisotropy, a large specific resistance, a high stability to an ultraviolet light, a high stability to heat, and so forth is especially desirable.